# Why do most dwarf planets have mass comparable to moon?

It was quite interesting to spot that most dwarf planets have masses close to that of our moon (if we let an error to fluctuate within two orders of magnitude).

Why it is so? Is there any common denominator of this phenomena? Maybe it's because all dwarf planets share similar formation roots/causes with the moon? Or is this just a very big and strange coincidence?

• Because dwarf planets has similar sizes to that of our moon. Apr 14 at 2:12
• It basically comes down to that being the range of objects we call "dwarf planets". Of course "dwarf planet" has a definition that doesn't depend on size, but imagine that Venus had been the size of Ceres and Ceres the size of Venus (but the rest of the asteroids still existed). Then it's unlikely that we'd have defined "dwarf planet" in the same way. Apr 14 at 9:25
• All stars are also within two orders of magnitude of 1 solar mass. All known rocky planets are within two orders of magnitude of one earth mass. All gas giant planets are within two orders of magnitude of one Jupiter mass. All naturally occurring chemical elements have atomic masses within two orders of magnitude of uranium. it's a huge range, on many classification scales. Apr 14 at 9:54
• I think it makes more sense to see it as the Moon being sized like a dwarf planet, rather than the reverse. It’s not unusual for dwarf planets to be in that size range; it is unusual for a planet’s satellite! Apr 14 at 15:14
• It's a pure selection effect. We just happened to name the most massive asteroids as dwarf planets. If you look up the mass distribution of asteroids or number distribution of asteroids then you will see that those are just the high-mass end of an (approximate) power-law distribution that goes down to meter-sized bodies. Apr 14 at 15:51

Two orders of magnitude is a very large range.

The Moon has a mass of $$7.342 \times 10^{22}$$ kg, so your question is, why do most dwarf planets have a mass between $$10^{20}$$ and $$10^{24}$$ kg?

By definition, a dwarf planet has to be in hydrostatic equilibrium. Considering a list of possible dwarf planets, the smallest/lightest for which there appears to be consensus that it is in hydrostatic equilibrium is 90482 Orcus, with a mass of $$(6.348 ± 0.019) \times 10^{20}$$ kg. Smaller bodies are too small to reach hydrostatic equilibrium. Hydrostatic equilibrium means that a body becomes spherical. Generally speaking, small solar system bodies are very far from spherical. It is not a requirement for a body to exist at all, but if it's not spherical, it's not considered a minor planet.

The Moon is also in hydrostatic equilibrium, which it would probably not be if it had less than 1% of its actual mass.

Now why are there no dwarf planets larger than $$10^{24}$$ kg? That exceeds the mass of Mars ($$6.4171 \times 10^{23}$$ kg) and approaches the mass of Earth ($$5.9724 \times 10^{24}$$ kg). Such large planets in the inner solar system are full planets and not dwarf planets. Such large bodies in the outer solar system have not been discovered and probably don't exist.

If the moon were two orders of magnitude larger, it would be as heavy as the Earth, and the Earth-Moon system would clearly be a double planet.

• Thanks for an answer. So in effect you are saying that dwarf planets are "the last man standing" at the edge of hydrostatic equilibrium ? Can be. But isn't it that hydrostatic equilibrium must be satisfied for ALL cosmic bodies origin, even for asteroids (smaller ones than dwarf planets) ? Because if inside pressure would not be in balance to gravity force - cosmic body would collapse or disintegrate, isn't it ? Or am I misunderstanding what hydrostatic equilibrium really mean ? Can you please expand a bit in your post what would happen to a body if it could not reach hydrostatic equilibrium ? Apr 14 at 7:48
• @AgniusVasiliauskas I've added a few lines explaining. Essentially, if a body is not in hydrostatic equilibrium, it's not spherical, and not considered a dwarf planet. Apr 14 at 8:37
• Regarding: "... the Earth-Moon joint centre of mass is not within the Earth", according to en.wikipedia.org/wiki/Barycenter, the barycenter of the earth-moon system is located inside the earth, at approximately 75% of its radius.
– Milo
Apr 14 at 10:24
• @Milo Thank you for the correction. Edited accordingly. Apr 14 at 12:16
• There's actually a proposal to classify planets by what they are (in hydrostatic equilibrium, below the mass of a star) instead of where they are or what they do ("clear their neighbourhood"). If the Earth was not in its orbit but the Moon still was, the Moon would be considered a planet, so why isn't it already? The Earth-Moon system should already be considered a double planet system. Apr 16 at 22:10

Consider tomatoes.... There are cherry tomatoes, salad tomatoes and beefsteak tomatoes.

Why do all salad tomatoes have about the same size (to within a couple of orders of magnitude)? Well if I have a tomato bush that produces smaller tomatoes then I call them "cherry tomatoes" and if my bush produces larger tomatoes, then I call them "beefsteak tomatoes". The fact that all salad tomatoes are about the same size is a consequence of my definition of "salad tomato"

It is the same with dwarf planets. They are defined to be in the middle range of masses for solar system bodies: larger than the asteroids, smaller than the planets. If a dwarf planet was much larger, it would be called a (major) planet, and if it was smaller it would be an asteroid, or TNO, or Kuiper belt object etc.

It is interesting that the moon also roughly the same mass. It seems too big, relative to the Earth! That suggests it didn't form in an accretion disc at the same time the the Earth formed (it didn't form in the same way that Io, Europa etc formed) Nor is it a captured asteroid (like Phobos) but it might have had some special mode of formation: the giant impact hypothesis.

• I understand what you are trying to say. But again, I don't think that all classes of tomatoes are likely to be produced or found in a store. Some may be more preferred than others. Same can be for planet sizes/masses. The reason why dwarfs are same scale as moon, can show that dwarf origin can be similar to moon origin. I'm not saying that all dwarf planets was formed by collisions/impacts between ordinary scale proto-planets, but clearly similar scales to moon must show some similar origins of them, and the reason of why dwarf planets are so popular in solar system. Apr 14 at 8:03
• That's right, but the point I'm making is that "salad tomato" is a human-defined category, based on size. In the same way "dwarf planet" is a human defined category, based on size. And so the reason that dwarf planets are the same size as each other is a result of their definition, not a coincidence or a result of a common mode of formation distinct from the other solar system bodies. Apr 14 at 8:21
• I understand that, however it would be interesting also to know why dwarf planet class is so frequent in solar system. Apr 15 at 6:16
• @AgniusVasiliauskas - if there was only one, there would not be a category. A category has been defined for those common things. You are thinking about this back to front. Apr 16 at 13:50

The currently accepted theory of how Luna (the Earth's moon) formed is the Giant Impact Hypothesis though there is still debate about that. Should that theory be true, it suggests that dwarf planets can be formed in orbit around major planets. Likely the same origin is the case for the moons of Jupiter and Saturn, though I'm not as familiar with their origin theories. Many different kinds of bodies can be formed in a solar system, but their resulting mass depends on where their original materials happened to be in space during their formation and the closest massive body around which they would eventually orbit.

Luna is not classified as a planet, but rather a moon, because it's orbiting the Earth. Ceres, which is smaller than Luna at ~$$10^{20}$$ kg, is a dwarf planet; it's orbiting our Sun and not another planet (dwarf or major). Titan, which is larger than Luna at ~$$10^{24}$$ kg is a moon; it's orbiting Saturn.

So, Luna is not called a dwarf planet simply because it's orbiting the Earth. If it wasn't, it would be a dwarf planet just like Ceres. Other answers have indicated the cutoff point between dwarf planets and major planets (and asteroids are smaller than dwarf planets). There's no coincidence in terms of mass, and possibly no difference in method of formation either, it's merely a classification difference due to its orbit.